Optical diagnostic monitoring of resonant-tunneling diode growth

We are employing in situ sensors to monitor and diagnose MBE growth in real-time. Two optical methods, spectroscopic ellipsometry (SE) and laser light scattering (LLS), provide complementary information on strained layer epitaxy. A quantum effect structure, the resonant-tunneling diode (RTD), provides a challenging and important test device vehicle for these studies. The RTD current-voltage characteristics exhibit sensitivities to thickness changes of less than a monoatomic layer. Strain relaxation in vertically-integrated RTD stacks was detected using LLS. Structural data determined using SE was correlated with RTD electrical characteristics (peak current, peak voltage, valley current, asymmetry ratios). Closed-loop control based on SE is being investigated to improve the reproducibility of RTD growth.